Noise as undesirable sound both in frequency and intensity has long plagued living and working environments. Actually, the elimination or reduction of sound can be quite complex. Several techniques have been employed such as redesigning or altering noise source, absorbing the sound by suitable materials, damping the sound by the use of sound-energy reducing materials, attenuating the sound by introducing barriers to sound transmission, and removing the source of the sound from the region affected. Of these, damping is probably the least understood and requires careful definition of the sound problem and the environment of use.
Viscoelastic materials are normally effective for sound damping. By definition, viscoelastic materials have a nonlinear response to stress. The graphic representation of the stress-strain relation of a viscoelastic material is a hysteresis loop in contrast with the straight line stress-strain response of a non-viscoelastic material. Viscoelastic materials are effective for damping because they can convert the cyclic kinetic energy of sound into other forms of energy, usually heat; note "Damping of sound energy with polymer systems", by W. H. Brueggemann, Modern Plastics, October, 1972, page 92.
However, the very fact that viscoelastic materials readily undergo damping vibration, ill equips such materials for structural applications. The damping response varies throughout a wide range of sound levels for different viscoelastic materials (reflected by the width of the hysteresis loop), such that the materials have poor dimensional stability and tend in time to creep. As a result, the practice has been to apply a viscoelastic material as a cover or coat to a structural member to reduce sound generated by it, much as one would paint such a member. This use has not always been satisfactory, however, for the coating of viscoelastic material tends to crack or flake off or otherwise become inoperative.
A commonly used measure of damping effect is "loss factor". When stress and strain are out of phase with respect to time, Young's modulus of the material is a complex quantity. The equation expressing this is: EQU E*=E.sub.1 +iE.sub.2
where E* is the complex modulus, E.sub.1 and E.sub.2 are the elastic modulus and loss modulus, respectively, and i is the square root of minus one. The ratio E.sub.2 /E.sub.1 is defined as the loss factor, n. The larger the magnitude of loss factor, the more effective is the damping material.
Temperature greatly affects the loss factor of a viscoelastic material. The loss factor increases and passes through a maximum as the temperature increases. When loss factor is plotted against temperature, a temperature range normally occurs in which the material has optimum damping characteristics and which is termed the glass transitional range. The temperature at which this range begins is termed the glass transitional temperature and is designated, Tg.